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Flow Dynamics of Two Phase (air and water ) Stratified Flow by a Simulation Package COMSOL MULTIPHYSICS
The figure showing different boundaries is
given in Appendix A.
Results without Surface Tension and
Gravity
The figure below is the result of flow of air
over water at rest but provided an outlet.
Basically in this flow problem the water is
shear driven, shear forces act at the interface
of flowing air and still water.
With no gravity and surface tension
coefficient, it is observed that when air flows
over water, the water level falls at start of the
water section. The fall in water section goes
approximately to the water wall contact,
forming slug, but the water level at the exit
of the water section remains constant. It was
expected since the cause of this pattern is
the absence of gravity, as in the absence of
gravity no external force acts on the flow and
the water level does not falls at the exit. At
the entrance when air just contacts the water
surface, the fall of water is due to drag, as
there is no inflow of water and, water flows
to the exit by interfacial shear force.
The pressure distribution is quite different
here (Appendix A). The pressure decreases
with the flow from left to the right in three
stages, in the beginning, in the middle and in
the end of flow. All pressure reductions takes
place suddenly. It means, no hydrostatic
pressure acts here.
Initially in the water section, water is at rest
with no inflow. When the flowing air with
velocity interacts with water, the direction of
the velocity changes to the down side due to
the water displacement and creation of wave.
Air moves the water with a little velocity. The
air gets maximum velocity at the top of the
wave due to pressure reduction by wave
generation.
Figure 5. Result of the flow when Gravity and
Surface tension are not considered.
Results with a Little Gravity and Surface
Tension
Here again it was tried to check the effect of
gravity(which must be taken into account) by
introducing a little value of Gravity 2m/s2 and
also by introducing Surface tension
coefficient. Here the result is a bit satisfactory
as the water level at the exit falls down mainly
due to gravity and somewhat due to shear.
As the water is shear driven, and is under
effect of gravity, the water level falls abruptly
at the exit and slowly at the start. If this goes
on, the water will be drained out of the section.
By introducing the gravity, the pressure
distribution is quite natural here, higher at the
bottom due to hydrostatic pressure of water
and lower at the top wall (air-wall contact).
(See appendix A) As the water falls down due
to gravity, a decrease in pressure takes place
above in the air zone and back flow of the air
occurs. The result just by introducing a value
of small gravity but keeping the surface
tension zero, the flow pattern is as when both
quantities were introduced.
When, only Surface tension coefficient was
introduced as in the first model, no effects
were observed. It just behaves as it does
without Surface tension coefficient.
Figure 6. The result when Gravity and Surface
tension are introduced.
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